Quality Attributes

Comprehensive definitions and guidance for quality attributes commonly used as ADR decision drivers.

Runtime Quality Attributes

Performance

Definition: How quickly the system responds and processes requests.

Measures:

Latency (response time in milliseconds)
Throughput (requests per second)
Resource utilization (CPU, memory, I/O)

Scenarios:

“The API must respond within 100ms for 95% of requests”
“The system must handle 10,000 concurrent users”

Trade-offs:

Often conflicts with maintainability (optimized code is complex)
May require more expensive infrastructure
Can conflict with flexibility

Availability

Definition: The proportion of time the system is operational.

Measures:

Uptime percentage (99.9%, 99.99%)
Mean Time Between Failures (MTBF)
Mean Time To Recovery (MTTR)

Common Targets:

Availability	Downtime/Year	Downtime/Month
99%	3.65 days	7.3 hours
99.9%	8.76 hours	43.8 minutes
99.99%	52.6 minutes	4.38 minutes
99.999%	5.26 minutes	26.3 seconds

Trade-offs:

Higher availability = higher cost
May require redundancy, which adds complexity
Can conflict with consistency (CAP theorem)

Scalability

Definition: Ability to handle increased load by adding resources.

Types:

Horizontal: Add more instances
Vertical: Add more resources to existing instances

Measures:

Maximum concurrent users
Data volume capacity
Scaling factor (2x users = ?x resources)

Trade-offs:

Horizontal scaling adds architectural complexity
May require stateless design
Increases infrastructure cost

Reliability

Definition: Ability to function correctly under stated conditions.

Measures:

Error rate
Data accuracy
Failure frequency

Aspects:

Fault tolerance: Continue operating despite failures
Recoverability: Restore operation after failure
Data integrity: Maintain data correctness

Security

Definition: Protection against unauthorized access and threats.

Aspects:

Confidentiality: Data accessible only to authorized parties
Integrity: Data not modified by unauthorized parties
Authentication: Verify identity of users/systems
Authorization: Control what authenticated users can do
Non-repudiation: Actions cannot be denied
Accountability: Actions can be traced

Measures:

Vulnerability count
Time to detect breach
Compliance certifications

Usability

Definition: How easy the system is to use effectively.

Measures:

Task completion time
Error rate
Learning curve
User satisfaction

Aspects:

Learnability
Efficiency
Memorability
Error prevention
Satisfaction

Development Quality Attributes

Maintainability

Definition: How easy the system is to modify and evolve.

Measures:

Cyclomatic complexity
Code coupling/cohesion
Technical debt metrics
Time to make typical changes

Aspects:

Modularity: Independent components
Analyzability: Easy to understand
Modifiability: Easy to change
Reusability: Components usable elsewhere

Testability

Definition: How easy the system is to test effectively.

Measures:

Test coverage
Test execution time
Defect detection rate

Factors affecting testability:

Dependency injection
Modular design
Observable state
Deterministic behavior

Deployability

Definition: How easy and safe it is to deploy changes.

Measures:

Deployment frequency
Deployment time
Rollback time
Failed deployment rate

Aspects:

Automation level
Rollback capability
Environment parity
Configuration management

Extensibility

Definition: How easy it is to add new capabilities.

Measures:

Time to add new feature
Impact on existing code
Plugin/extension support

Factors:

Plugin architecture
Well-defined interfaces
Loose coupling

Operational Quality Attributes

Observability

Definition: Ability to understand system state from external outputs.

Pillars:

Metrics: Quantitative measurements
Logs: Event records
Traces: Request flow tracking

Measures:

Time to detect issues
Time to diagnose root cause
Alert accuracy

Manageability

Definition: How easy the system is to operate and administer.

Aspects:

Configuration management
Monitoring and alerting
Backup and recovery
User administration

Portability

Definition: Ability to run on different platforms/environments.

Types:

Platform portability: Different OS/hardware
Cloud portability: Different cloud providers
Data portability: Move data between systems

Measures:

Platforms supported
Migration effort
Environment-specific code percentage

Interoperability

Definition: Ability to work with other systems.

Aspects:

Standard protocols
API compatibility
Data format support
Integration patterns

Business Quality Attributes

Cost Efficiency

Definition: Value delivered relative to cost.

Measures:

Total cost of ownership (TCO)
Cost per transaction
Infrastructure cost
Development cost

Time to Market

Definition: Speed of delivering new capabilities.

Measures:

Feature lead time
Release frequency
Time from idea to production

Compliance

Definition: Adherence to regulations and standards.

Common Requirements:

GDPR (data privacy)
HIPAA (healthcare)
PCI-DSS (payment cards)
SOC 2 (service organizations)
ISO 27001 (information security)

Quality Attribute Trade-off Matrix

Attribute	Conflicts With	Because
Performance	Maintainability	Optimizations add complexity
Performance	Portability	Platform-specific optimizations
Security	Usability	More security = more friction
Security	Performance	Encryption/validation overhead
Availability	Cost	Redundancy is expensive
Availability	Consistency	CAP theorem
Scalability	Simplicity	Distributed systems are complex
Flexibility	Performance	Abstractions have overhead
Flexibility	Simplicity	More options = more complexity

Using Quality Attributes in ADRs

In Decision Drivers Section

## Decision Drivers

* **High availability (99.99%)** - SLA requires < 1 hour downtime/year
* **Horizontal scalability** - Must handle 10x current load by Q4
* **Maintainability** - Small team must support long-term
* **Security compliance** - Must meet SOC 2 requirements

In Options Evaluation

### Option 1: Managed Service

* **Availability**: Excellent - provider guarantees 99.99%
* **Scalability**: Good - auto-scaling included
* **Maintainability**: Excellent - no infrastructure to manage
* **Cost**: Moderate - $X/month at current scale
* **Security**: Good - provider is SOC 2 certified

In Consequences Section

## Consequences

* Good, because availability target will be met with provider SLA
* Good, because team can focus on features instead of infrastructure
* Bad, because we accept vendor lock-in
* Bad, because cost will increase with scale